trying to set assignee based on custom field via script runner

using built in scripts : setting custom field value to another field against the filter i have set however i cannot figure out the correct id for 'assignee' i have tried just assignee however im not sure if i need to set it to something like system_assignee etc.. does anyone know?

not wanting to do it on post function just wanting to run a clean up script.

I’m a designer on the Jira team. For a long time, I’ve fielded questions from other designers about how they should be using Jira Software with their design team. I’ve also heard feedback from other ...